ERK kinases are serine/threonine kinases that mediate intracellular signal transduction pathways involved in tumor growth, progression and metastasis. ERK is involved in the Ras/Raf/MEK/ERK pathway, which plays a central role in regulating cellular processes by relaying extracellular signals from ligand-bound cell surface receptor tyrosine kinases (RTKs) such as ErbB (e.g. EGFR, Her-2, etc), VEGF, PDGF, and FGF receptor tyrosine kinases. Activation of an RTK triggers a series of phosphorylation events, beginning with the activation of Ras, followed by recruitment and activation of Raf. Activated Raf then phosphorylates MAP kinase kinase (MEK) 1/2, which then phosphorylates ERK 1/2. ERK phosphorylation by MEK occurs on Y204 and T202 for ERK1 and Y185 and T183 for ERK2 (Ahn et al., Methods in Enzymology 2001, 332, 417-431). Phosphorylated ERK dimerizes and translocates to and accumulates in the nucleus (Khokhlatchev et al., Cell 1998, 93, 605-615). In the nucleus, ERK is involved in several important cellular functions, including but not limited to nuclear transport, signal transduction, DNA repair, nucleosome assembly and translocation, and mRNA processing and translation (Ahn et al., Molecular Cell 2000, 6, 1343-1354). ERK2 phosphorylates a multitude of regulatory proteins, including the protein kinases Rsk90 and MAPKAP2 ((Bjorbaek et al., 1995, J. Biol. Chem. 270, 18848; Rouse et al., 1994, Cell 78, 1027), and transcription factors such as ATF2, Elk-1, c-Fos, and c-Myc (Raingeaud et al., 1996, Mol. Cell Biol. 16, 1247; Chen et al., 1993, Proc. Natl. Acad. Sci. U.S.A. 90, 10952; Oliver et al., 1995, Proc. Soc. Exp. Biol. Med. 210, 162). Overall, treatment of cells with growth factors leads to the activation of ERK1 and ERK2 which results in proliferation and, in some cases, differentiation (Lewis et al., Adv. Cancer Res. 1998, 74, 49-139).
A wealth of studies have shown that genetic mutations and/or overexpression of protein kinases in the Ras/Raf/MEK/ERK pathway lead to uncontrolled cell proliferation and tumor formation in proliferative diseases such as cancer. For example, some cancers contain mutations which result in the continuous activation of this pathway due to continuous production of growth factors. Other mutations can lead to defects in the deactivation of the activated GTP-bound Ras complex, again resulting in activation of the MAP kinase pathway. Mutated, oncogenic forms of Ras are found in 50% of colon and >90% pancreatic cancers as well as many others types of cancers (Kohl et al., Science 1993, 260, 1834-1837). Recently, bRaf mutations have been identified in more than malignant melanomas (60%), thyroid cancers (greater than 40%) and colorectal cancers. These mutations in bRaf result in a constitutively active Ras/Raf/MEK/ERK kinase cascade. Studies of primary tumor samples and cell lines have also shown constitutive or overactivation of the Ras/Raf/MEK/ERK kinase pathway in cancers of pancreas, colon, lung, ovary and kidney (Hoshino, R. et al., Oncogene 1999, 18, 813-822). Further, ERK2 has been shown to play a role in the negative growth control of breast cancer cells (Frey and Mulder, 1997, Cancer Res. 57, 628) and hyperexpression of ERK2 in human breast cancer has been reported (Sivaraman et al., 1997, J Clin. Invest. 99, 1478). Activated ERK2 has also been implicated in the proliferation of endothelin-stimulated airway smooth muscle cells, suggesting a role for this kinase in asthma (Whelchel et al., 1997, Am. J. Respir. Cell Mol. Biol. 16, 589).
In view of the multitude of upstream (e.g. Ras, Raf) and downstream (e.g. ATF2, c-Fos, c-Myc) signaling proteins in the Raf/Ras/MEK/ERK pathway that have been implicated in a wide range of disorders, including but not limited to cancer, ERK has emerged as a prime target for drug development.